490 THE FOKxAlS OF TISSUES [ch. 



In the four-celled stage of the frog's egg, Rauber (an exception- 

 ally careful observer) shews us three alternative modes in which 

 the four cells may be found to be conjoined (Fig. 175). In A we 

 have the commonest arrangement, which is that which we have 

 just studied and found to be the simplest theoretical one; that 

 namely where a straight polar furrow intervenes, and where the 

 partition- walls are conjoined at its extremities, three by three. 

 In B, we have again a polar furrow, which is now seen to be a 

 portion of the first "segmentation-furrow" by which the egg was 

 originally divided into two; the four-celled stage being reached by 

 the appearance of the two transverse furrows. In this case, the 

 polar furrow is seen to be sinuously curved, and Rauber tells us that 

 its curvature gradually alters; as a matter of fact, it, or rather the 



-f- 



Fig. 175. Various conjunctions of the first four cells in a 

 frog's egg. After Rauber. 



partition-wall corresponding to it, is gradually setting itself into a 

 position of equilibrium, that is to say of equiangular contact with 

 its neighbours, which position is already attained or nearly so in 

 A. In C we have a very different condition, with which we shall 

 deal in a moment. 



The polar furrow may be longer or shorter, and it may be so 

 minute as to be not easily discernible; but it is quite certain that 

 no simple and homogeneous system of fluid films such as we 'are 

 deahng with is in equilibrium without its presence. In the accounts 

 given, however, by embryologists of the segmentation of the egg, 

 while the polar furrow is depicted in the great majority of cases, 

 there are others in which it has not been seen and some in which 

 its absence is definitely asserted*. The cases where four cells lying 



* Thus Wilson declared (Journ. Morph. viii, 1895) that in Amphioxus the polar 

 furrow was occasionally absent, and Driesch took occasion *to criticise and to 

 throw doubt upon the statement {Arch. f. Entw. Mech. i, p. 418, 1895). 



